This invention relates to a ring furnace for producing carbon bodies for the electrometallurgical industries, and refers, more specifically, to a suction tube assembly for removing hot packing coke from a chamber of such furnace.
The carbon bodies which are used in cells for the electrolytic production of aluminum are made of a mixture, or a paste, which consists of calcined anthracite, petroleum coke and pitch, in combinations varying with the raw materials and the purpose of the body. Pitch is used as a binder.
Carbon bodies are also used in furnaces for electrothermal processes, in linings and as electrodes. For making these bodies, other mixtures are used, but the principles are the same.
After mixing in a mixing machine, this paste or mixture is firm at room temperature, becomes softer over 100.degree.-150.degree. C., and at higher temperatures and volatile components are driven off, and the binder becomes carbonized, whereupon the paste becomes rigid and hard. This thermal treatment is known as baking. It can be effected in the reduction cell itself, in that the heat developed therein bakes the filled carbon paste. However, in more recent times, it is preferred to perform this baking in separate furnaces, since this enables better control of the baking process and thereby the properties of the finished product.
The carbon bodies which are put in such a furnace for baking are often referred to as "green" carbons, the word "green" implying that they are not "ripe". They are produced by pressure or by vibration. The green carbons can be made to be of considerable size. Cathode carbon blocks, for example, may measure 700 mm.times.900 mm.times.4,000 mm, and weight about 4 tons. It is clear that when such a body is heated up and passes through a softening temperature zone, it will become deformed unless special steps are taken. The green carbons and therefore placed in high pits in the furnace which are built of refractory materials, the space between the green carbons and the walls of the pit being filled with coke breeze, i.e. small pieces of coke. This coke breeze also protects the green carbons from combustion.
A given number of pits form a chamber, and a number of chambers are built together to form a ring furnace with a separate cover over each chamber. Hot combustion gasses are conducted into a chamber of such furnace, flow through hollow refractory pit walls and then on to the next chamber. The firing zone wanders through the whole furnace in order thereby to make best possible use of the heat. The baking of the green carbons is completed sequentially and the chambers containing the finished carbons must be left to cool for some time before they can be emptied of packing coke and the baked carbon bodies lifted out. During the baking process, the temperatures reaches 1,280.degree. C., and the entire cycle takes some three weeks.
Ring furnaces have large dimensions. Cranes over the furnace may, for example, have a span of some 30 m. The installations are costly and it is therefore necessary to make efficient use of the heat energy, and to reduce labour time by means of expedient aids.
The physical work in operating ring furnaces consists mainly of emptying the chambers at the end of a cycle and refilling them with green carbons and packing coke for the next cycle.
The emptying of an individual chamber consists of lifting off the lid and then sucking up the packing coke in order to expose the baked carbons. Suction is the most practical method of removing the coke breeze.
The sucking up of the coke breeze is usually performed by lowering a suction tube down into the packing coke. This tube is usually suspended by a crane, and telescopically connected with a silo and suction coupling on the crane bridge. The tube is lowered down into the loose coke mainly by its own weight, and is often guided manually by handles welded on the tube. The operator concerned must therefore sit on the side wall of of a chamber over the hot coke, and is exposed to heat and dust for the considerable length of time it can take to empty a chamber.
The method of operating the suction tube is thus laborious and not entirely efficient. If the end of the tube penetrates too far down into the coke, insufficient air is sucked in for the satisfactory transportation of the coke in the tube, with partial or complete stoppage being the result.